It has not been easy to make high-gain antennas that can re-aim themselves in just tens of nanoseconds.
Over a century ago, antenna designers devised methods to create high-gain antennas. A typical class of high-gain antenna might employ a parabolic reflector. In this class of antenna, an RF feed point would be located at the focus of the parabola, pointed toward the reflector. The size of the reflector would then be proportional to the effective size of the antenna, resulting in higher gain.
Re-aiming such an antenna from one point in the sky to a different point in the sky would require physically re-aiming the antenna. Several factors impose limits on how quickly such a re-aiming could be carried out. A first factor, relating to the rotational moment of the antenna, is the available torque in the servomotors that rotate the antenna. A second factor is that even if the servomotors are arbitrarily strong, the antenna might not be strong enough to resist deformation when accelerated. It can take hundreds of milliseconds or even longer than a second to re-aim a parabolic reflector antenna from one point in the sky to a different point in the sky.
Some decades ago, Electronically Scanned Antennas (ESAs) were developed. With an ESA, many individual antenna elements are fed through some means of delaying the electrical signal to each, typically through the use of a phase shifter (as in a phased-array antenna). Each antenna element is fed through a respective phase shifter, which is programmable to inject a desired amount of delay (typically discretized into several bits per phase shifter). The task of aiming such an antenna includes sending control signals to the phase shifters to configure the intended delay per element.
For transmission, an RF signal is injected into the common port of the phased array antenna and then divided up and propagated to the elemental phase shifters. Each phase shifter propagates the signal to its respective antenna element after applying its commanded phase shift to the signal. By design, the energy radiated by each antenna element adds in free space to yield a focused beam in a particular direction. In a similar way, such an antenna can serve as a receive antenna with gain in a particular direction.
As compared with a parabolic reflector antenna, a phased-array antenna offers the benefit of being able to re-aim the beam just by reconfiguring the phase shifters and without a need to physically re-aim the antenna. The phased-array antenna has a drawback of cost and complexity due to the presence of hundreds or thousands of phase shifters (typically one per each antenna element in the array). Each phase shifter itself also takes up some physical volume and this entails that the antenna will have some bulk. Additionally, the inherent losses of phase shifters typically require that additional signal amplification is provided at each element further increasing the complexity, power draw, and thermal-management requirements.
The time required to re-aim such an antenna might be on the order of 100 nanoseconds (ns) to one microsecond (μs) or longer.
With a single-beam high-gain antenna, by definition at a time when the antenna is pointed in some particular direction, the antenna is not available to be pointed in any other particular direction. It will often be desired that an antenna will scan with two degrees of freedom to look for points of interest in a region. If a point of interest is detected, closer scrutiny in the direction of the point of interest may be required. With many prior-art antennas, it is difficult or impossible to continue monitoring in other directions in a way that can be interleaved with closer scrutiny in a particular direction.
Suppose, for sake of discussion, that some point of interest is detected within the field of view (FOV) of an antenna, defined, for example, by a direction from which there is a return signal. It might be desirable to dither the aim of the antenna, meaning to change the aim of the antenna slightly in each of several directions, to see whether the strength of the return signal increases or diminishes. The goal is, of course, to identify the specific direction in which the signal is the strongest. With a parabolic reflector antenna, such dithering requires physical movement of the antenna and so takes some time to settle. With a phased-array antenna, such dithering does not require physical movement of the antenna, but nonetheless takes some time.
Every high-gain antenna, no matter how cleverly designed and no matter how carefully manufactured, generates side lobes. For an antenna that is transmitting, by “side lobes” is meant that while most of the RF energy propagates in some particular direction (the intended direction of high gain), some of the energy also propagates in other directions. For an antenna that is receiving, by “side lobes” is meant that the antenna picks up RF energy not only from the direction of high gain but also, to some extent, from other directions.